Kristof Indeherberge wrote:Extra points if you can predict what can happen further down the stream if you merge it with another layer (depending on what you're doing). Take a good look at that curve.

I'm not sure what you mean. We're in SLog2 for three projects right now and not having any trouble. At least, not with the log to lin part of the transform. Yes, it goes super-white a little early, but assuming the DP is monitoring properly, things generally work out. If we're comping a bright element and need to match the white level, we'll gain down first, then back up after the merge in order to make the match easier.

michael vorberg wrote:I guess you can create negative values in the blacks

That's true. ARRI LogC, which we're accustomed to using for most shows, does the same thing. Assuming you never clip black on the plate, it's usually okay. In the event that we light a CG render with the plate, we add a small brightness value before making the render background EXR. Otherwise, Redshift shows bright pink pixels in reflections.

I keep meaning to dig into color science a little deeper and verify Fusion's log conversions against the ones published by the manufacturers. Too many projects…

It creates negative values, and more importantly negative values as canvas colour. So if you transform the plate, making it smaller, you will compromise your background input when merging. Causing some head scratching for sure.

Preview the merge in the buffer, and toggle it while keeping an eye on the green background.

I do think this should be addressed by BMD. And it is not negligible. It will happen more often than you'd think, especially when keying and transforming plates.

One way to approach this is to lift the linear values by a small amount to keep noise floor and clip at 0.0 to avoid negatives. Then in the end subtract the offset and convert to log again if necessary.

The overall logic should be that linear value of zero is at the middle of black cap noise (noise values should average to zero) when considering the black level of log curve. Log data can keep the sub-blacks as positive values because average black is not at zero but in linear they are negative.

Slog2 (or, even worse, Slog3) is a very flat curve, meant for high contrast scenes to capture all values the sensor can differentiate. It was initially developed for professional cameras, which record 422 with at least 10 bit.

Unfortunately, on a camera recording in heavily compressed 8 bit, like the Alpha series, it's not much more than a marketing gimmick. Don't use it for greenscreen! Don't even record internally, but use a decent external recorder like an Atomos or BM.

Since green screen work is normally a controlled lighting situation, you can always limit your scene contrast to something which is easily captured with a profile like Cine 1, 2 or 4. These offer superwhites, which will give you another 2/3 of a stop in the highlights (Cine 2 doesn't). Just compare them and use what you like, as long as you keep the green screen around 50-60% on the waveform.

The basic approach with any 8 bit recording should be getting as close as possible to the intended final look, since you'll need to avoid any heavy grading.

- record to an external recorder - in my case the BMD Video assist 4k which bypasses the compression of the camera – with a 10 bit codec ProresHQ- and use a Cine Profile 1 or 4 to preserve the superwhites- light the greenscreen to 50-60%

Keep in mind that if you want to retain the log style for grading after VFX, you'll need a way to invert the Log to Lin conversion. In Nuke you literally bring in the footage, apply "LogToLin", dial in your settings, do the comp, and at the end before rendering it out, you copy the first LogToLin and invert it. Everything goes Log, including the effects. Makes grading a lot easier since the VFX shot matches the rest.

Eh...I don't know Nuke, so maybe I don't quite understand what you're saying, but if you do LogToLin, it sounds like you convert TO Lineair, do your comp and *then* convert back to log. So you do your comping in lineair. You would do this the same in Fusion. I've done it on Cineon-log files back in 2007.This workflow hasn't changed in that sense.